Prior art comparators typically utilize bipolar transistors for active devices. It is characteristic of such comparators to have relatively low input impedance and high input offset and bias currents. When operated from a single positive supply voltage, such comparators exhibit poor operation as the input common mode (reference) voltage approaches ground. Also, a typical one of these circuits cannot operate with a negative input common mode voltage.
In using the techniques described in greater detail hereinafter, it is possible to design a complete comparator circuit using only MOSFETs. No other type of components is needed in one such configuration. Another advantage of a comparator having a MOSFET differential amplifier over existing differential amplifiers is the extremely large input impedance which is characteristic of the gate to source impedance of MOSFETs. Since the MOSFET is a voltage driven device as contrasted to bipolar transistors which are current driven devices, extremely low offset and bias currents can be realized using MOSFETs for input transistors. A MOSFET differential amplifier exhibits a leakage current lying in the range of pico amps rather than in the range of nano amps as found in bipolar amplifiers.
With proper control of the threshold voltages of both the P and N-channel transistors, a CMOS differential amplifier operates with an extremely large input common mode voltage swing and thus allowing the comparator to operate over a wide range of reference voltages. When such an amplifier operates from a single positive supply voltage, it can operate with negative input common mode voltage signals. Such an operating mode cannot be easily obtained from bipolar amplifiers since V.sub.BE of both the NPN and PNP transistors is virtually equal.
Another advantage of a comparator using a CMOS differential amplifier is that true complementary symmetry can be obtained with a CMOS integrated circuit, while true complementary symmetry cannot be obtained when using bipolar transistors. It is common knowledge that good PNP transistors cannot be made in the same process as good NPN transistors. However, a good P-channel device can be made in the same process as a good N-channel device. Although a MOSFET device is considerably smaller than a bipolar transistor, comfortable gains are obtainable from these CMOS differential amplifiers.
The comparators described in this invention used CMOS differential amplifiers having MOSFETs for active loads. This characteristic offers the following advantages:
A. It makes it possible to design a comparator circuit with MOSFETs being the only components used.
B. It provides large load impedances and, hence, large voltage gain with relatively small chip area by increasing gate width(s) and operating the load devices in the saturation region.
c. Utilizing the MOS process technology, active devices can be fabricated to have a better match than passive components such as resistors. Therefore, using MOSFETs for active loads improves the input offset voltage characteristics of the differential amplifier. Since large load impedances are realized in small chip areas, low parasitic capacitances are associated with these loads which increases the amplifier's slewing rate, which improves the switching speed of the comparator and, hence, the range of reference voltages.
d. Input common-mode voltage swings can be improved by controlling the threshold voltages of the load devices.
A relatively high initial input offset voltage is characteristic of a CMOS differential amplifier as compared to a low input offset voltage as being characteristic of a bipolar differential amplifier. Statistical data obtained on recently integrated CMOS differential amplifiers have shown that input offset voltage drift with temperature is as good as those obtained from some bipolar transistors.
Various designs of CMOS comparators are described hereinafter that can be operated from either a single positive supply voltage or a single negative power supply voltage. It is possible to design a monolithically integrable CMOS comparator that can offer an extremely low power drain level and/or an extremely wide range of supply voltages.